The invention relates to an RF resonator for an NMR apparatus, wherein the RF resonator comprises a birdcage resonator for transmitting and/or receiving signals with a first measurement frequency, wherein the RF resonator is symmetrical with respect to an xz-plane and a yz-plane of a Cartesian coordinate system with a Z-axis, with at least two electrically conducting ring elements that are disposed coaxially about the Z-axis and spaced from each other, N electrically conducting bars that are disposed parallel with the Z-axis, where N>4 and is even, wherein each ring element is electrically connected with all N bars.
Such an RF resonator is known from reference [1].
A birdcage resonator comprises N bars and at least 2 ring elements. A bar is understood to be electrically conducting elements elongated in the z-direction that usually lie approximately on a cylinder envelope parallel with the z-axis and that have an upper and a lower end.
Ring elements are understood to be electrically conducting ring or tube-shaped, usually circular or elliptical at least in sections, that are electrically (galvanically or capacitively) connected with N bars at the upper or lower end.
Moreover, a birdcage resonator has capacitors that can interrupt the bars, the ring elements, or both. At least one capacitive interruption must exist, either on at least one of the ring elements between each connecting point of a bar with ring elements or on each bar between the two connecting points of the bars with the ring elements. For capacitive interruption of the bars of a birdcage resonator, therefore N capacitors, for capacitive interruption of a ring element, at least N−2 capacitors are required.
A birdcage resonator has two orthogonal linearly polarized modes that generate a dipolar field that can be conveniently used for NMR. In a low-pass birdcage resonator, these are the two lowest modes.
FIG. 1b shows the mode spectrum with two linearly polarized modes 14 of a symmetrical birdcage resonator with eight bars, as is shown in FIG. 1a. If the birdcage resonator is rotationally symmetric, these two linearly polarized modes 14 are degenerate and can be used simultaneously for quadrature operation at one frequency to produce a circularly polarized field. The arrow in FIG. 1b marks the two degenerate linearly polarized modes 14 that oscillate at a frequency F1. The advantage of quadrature detection and excitation are a signal-to-noise ratio that is √2 higher than with detection/excitation with linearly polarized modes and is achieved in particular even with measurement samples subject to losses. Furthermore, with quadrature excitation, only half the power is required for the same flip angle and the same pulse length.
For quadrature operation of a resonator, two mutually orthogonal orientations by means of coupling networks are usually preferred and thus form two mutually orthogonal mirror symmetries along the xz- and the yz-plane. To generate and detect the circular polarization, the linearly polarized modes are operated and detected with a phase difference of 90°. The relevant modes are thus mutually orthogonal, i.e. the electrical plane of symmetry of the first degenerate mode corresponds to the magnetic plane of symmetry of the second degenerate mode and vice versa.
FIG. 1a shows a discrete element circuit diagram of such a low-pass birdcage resonator with eight bars 11 that are electrically connected to two ring elements 12. The bars are capacitively interrupted by sixteen capacitors 13 with capacitances C1-C8. The inductive couplings between bars 11, the ring elements 12, and between bars and ring elements are not drawn in FIG. 1a. 
To tune more than only one frequency that can be used for NMR on a birdcage resonator, it is possible to break the symmetry of the birdcage resonator (e.g. by shifting the geometric positions of the bars, varying the capacitance of the capacitors, or a combination of the two) and thus generating two orthogonal modes of different frequencies F1 and F2, as is disclosed in reference [1], p. 415. This usually changes the capacitances in such a way that, instead of standard values C of all capacitors, e.g. the capacitance values of two capacitors in each case are lowered (C1=C5<C) while those of two other capacitors are increased (C3=C7>C). The capacitance values of the remaining four capacitors remain unchanged (C2=C4=C6=C8=C). This cancels the degeneration of the linear modes and shifts one to a higher, the other to a lower frequency. The mode spectrum of such a non-symmetrical birdcage resonator is shown in FIG. 2a. This method is typically used to split the resonance for frequencies that are relatively close to each other. This is the case, for example, of 1H and 19F. By splitting the symmetry, the degeneration of the other modes is canceled so that the birdcage resonator now has a total of eight modes at different frequencies, wherein only the lowest two generate a dipolar field that can be conveniently used for NMR (linearly polarized). These modes are marked with arrows in FIG. 2a. 
In this first variant of a doubly tuned birdcage resonator known according to the prior art, the bars and ring element of the birdcage resonator are identical for both frequencies, i.e. both frequencies oscillate on the same bars and ring elements. In this way, the efficiency of the resonator is only slightly impaired at both measurement frequencies by the presence of the second frequency. The field profiles are almost identical so that both frequencies “see” exactly the same measurement volume. However, the two modes can only be operated linearly, i.e. quadrature operation is no longer possible!
If the frequency difference between the modes is too great, the field homogeneities of both modes become severely impaired in this first variant because the currents are no longer evenly sinusoidally distributed over the bars. Field homogeneity is understood here, in particular, to be the radial distribution of the field whereas field profile is understood to be the axial distribution. The quality and therefore also the efficiency are further reduced because the current only uses some of the bars. The remaining conductors are “in the way” of the field, generate additional losses and partially shield the field in the measurement volume. In the extreme case of very different frequencies, the current flows almost only on two of the eight bars in both resonance modes.
In this case, it makes sense to build a second variant of a doubly tuned birdcage resonator, in which the birdcage resonator comprises two sub-grids with capacitors with different capacitance values C2=C4=C6=C8=Ca and C1=C3=C5=C7=Cb, where Ca<<Cb, as is disclosed in reference [1], p. 417. In this birdcage resonator, two degenerate modes arise at each of the frequencies F1 and F2. In a birdcage resonator with a total of eight bars, the field homogeneity of these two modes can only be used in quadrature operation during excitation and reception (as for a resonator with only four bars). The spectrum of such a birdcage resonator is shown in FIG. 2b; each of the two degenerate linearly polarized modes is indicated by an arrow.
The situation of two measurement frequencies that are far apart occurs in nuclear magnetic resonance spectroscopy, in particular, if F1 is the frequency of 1H or 19F and F2 of a nucleus of low frequency, in particular, 31P, 13C, or 15N.
For operation with only linear polarization, the homogeneity is also extremely poor in the second known variant and the number of bars must be at least doubled. Further, the lower frequency partially shields the higher frequency so that the efficiency of the resonator is severely reduced as compared with the symmetrical resonator. The shielding effect increases as the number of bars increases while a large number of bars is essential for good field homogeneity.
In this second variant of the double tuned birdcage resonator with two sub-grids, the ring elements are used at both measurement frequencies while the bars are only used at one. This results in losses because the unused bars are “in the way” of the RF field and take almost no net current to resonance.
A subclass of these is formed by birdcage resonators in which the legs oscillating at the high frequency are insulated from the influence of the lower frequency by bandstop filters, thus improving the efficiency at the higher frequency to the detriment of the efficiency at the lower frequency.
A third variant of a doubly tuned birdcage resonator is described using multiple end rings, as described, for example, in reference [4]. There are combinations of low-pass and high-pass versions. However, in all of them, the higher frequency always has to be blocked in the ring element of the lower frequency by means of switches or low-pass filters. These filters or switches produce additional losses or noise so that the efficiency is reduced, in particular, at the higher frequency. Due to the larger number of rings, the field profile is usually no longer identical at the different frequencies. However, this is not particularly critical in practice. Here, the field profile is understood to be the field distribution in the measurement sample as a function of z.
In birdcage resonators with multiple end rings, the two resonance frequencies can each be tuned to two degenerate modes and thereby used in quadrature. The spectrum of such a birdcage resonator very much depends on the specific technical implementation, so it is not described here.
Furthermore, according to the third variant, a birdcage resonator is extremely complex to manufacture because the bandstop filters must be tuned identically, must not generate an NMR-active field (i.e. no field components that are perpendicular to the static magnetic field B0 in the measurement volume), must not produce interference with the homogeneity of the static magnetic field B0 due to their magnetic susceptibility, and the symmetry must not be broken during tuning (i.e. during adjustment of the resonance frequency of the resonator to the Larmor frequency of the nucleus to be measured in the given static magnetic field B0) and when the resonator is coupled. Because of the numerous technical difficulties, this concept has not become generally accepted in NMR.
Because of the numerous different ways of implementing such birdcage resonators with integrated filters, reference is made to references [1], [3], [4] and these are not described or explained in further detail here.
The object of the invention is to propose an RF resonator for a magnetic resonance measuring head that is optimized for transmitting and receiving at two measurement frequencies, wherein one of the frequencies is clearly higher than the second frequency, in particular, with a frequency difference of more than 7%. The field homogeneity and efficiency should be maximum for both frequencies and the field profiles, in particular, the length in the z-direction and the rate of change (decrease in the z-direction at the edge of the field profile) should be identical, if possible.